1. Field of the Invention
This invention pertains to non-invasive monitors, and more particularly to non-invasive monitoring of cardiac function.
2. Prior Art
The prior art teaches transducers placed on the neck for detecting internal jugular and carotid pulse waveforms. See, for example, Tavel, Clinical Phonocardiography and External Pulse Recording, Year Book Medical Publishers, Inc., 3d Ed., 1978, pp. 25-32. Transducers placed on the neck have also been used by the present inventor for obtaining carotid pulse waveforms and for monitoring respiratory events, as described in U. S. Pat. No. 4,452,252, which suggests use of a particular transducer, denominated the neck inductive plethysmograph. This patent, at column 6, also suggests placing two such transducers about the neck for obtaining both respiratory and cardiac waveforms, depending upon the type of filtering used.
While the cardiac information derived from such transducers is useful, it is not substitutable for venous pressure measurements. Venous pressure measurements are a widely recognized diagnostic tool for assessing cardiac function. Elevation of venous pressure is common in congestive heart failure and may precede other signs such as peripheral edema, ascites, pleural effusion and liver enlargement. For obvious reasons, venous pressure trends are more valuable than a single value of the absolute venous pressure level.
Measurement of peripheral venous pressures have long been utilized to follow the course of congestive heart failure, but peripheral pressures may be altered by local venoconstriction. Therefore, measurement of venous pressure within the central venous system, i.e. superior vena cava, has become the preferred site of measurement for assessing overall hemodynamic function, and numerous investigations have confirmed the clinical usefulness of central venous pressure (CVP) for hemodynamic monitoring. Further, CVP generally reflects the degree of hypovolemia (low circulating blood volume) thereby providing a guide to fluid replacement therapy.
A review of the invasive and visual methods for determination of peripheral venous pressures is found in Burch, A Primer of Venous Pressure, Lea & Febiger, 1950. For both invasive and visual methods, errors due to improper selection of a reference level for the influence of gravity need to be considered. In order to overcome such errors, a point in the venous system must be utilized at which the venous pressure is equal to that of the atmosphere. Since none of the points of reference is always at "zero" or atmospheric pressure, it is important to employ a satisfactory standard level of reference which all observers can utilize to obtain comparative values. Burch, supra, selected the phlebostatic axis or "heart level"--defined as the line of junction between (1) a transverse plane of the body passing through the points of junction of the lateral margins of the sternum and the fourth intercostal space of the rib cage and (2) a frontal plane of the body passing through the midpoint of a line extending from the outermost point of the anterior surface of the sternum and the outermost point of the posterior surface of the chest--as a satisfactory reference point for venous pressures.
Many techniques have been advocated for invasively measuring the venous pressure in peripheral veins. Burch, supra. Trends in such pressures generally agree with central venous pressures (CVP) in the high ranges but diverge in the low ranges. In hypovolemia (low circulating blood volume), peripheral venous pressures largely reflect local influences upstream from their site of measurement and inaccurately estimate the degree of hypovolemia. See Shoemaker et al., Textbook of Critical Care, W. B. Saunders Company, 1984; Rippe et al., Intensive Care Medicine, Little Brown and Company, 1985.
For direct measurement of CVP, invasive CVP monitoring (continuous measurement of superior vena cava pressure) by insertion of central venous catheters is routinely employed, and CVP derived from such catheters is used to guide fluid therapy after hemorrhage, accidental and surgical trauma, sepsis, and emergency conditions associated with blood volume deficits Shoemaker, supra; Rippe, supra.
A healthy ambulatory person in the supine posture may have CVP values up to about 6 cmH.sub.2 O. Ten to 12 cmH.sub.2 O is frequently used as the upper limit of normal for acutely ill patients. However, critically ill patients receiving mechanical ventilation and positive end-expiratory pressure (PEEP), who require fluid volume to maintain arterial pressure, may develop CVP values of 20 to 25 cmH.sub.2 O. Shoemaker, supra; Rippe, supra.
CVP measurements are highly useful during early resuscitation from acute cardiac injury Increased CVP usually indicates that fluids have been administered too rapidly. CVP is most helpful when there is failure of only one organ system, such as cardiac failure or uncomplicated blood loss. Change in CVP in response to a fluid volume challenge over a prescribed time period reflects the reserve capacity of the heart.
From a simplistic point of view, the CVP level appears to be determined by blood volume and right heart function. Large fluid infusions produce only small changes of CVP in the hypovolemic patient, but increased blood volume elevates CVP in the overtransfused stressed patient. Fluid overload precipitously increases CVP in patients with cardiac disease. However, despite the apparently simple relation between CVP and blood volume, it is inadvisable to rely solely on CVP as a reliable measure of circulating blood volume, as there are several factors which influence CVP values, not the least of which are cardiac performance, blood volume and vascular tone, intrinsic venous tone, increased intra-abdominal or intrathoracic pressures, and vasopressor therapy. Shoemaker, supra; Rippe, supra.
Four sites are commonly utilized for insertion of central venous catheters: the antecubital veins, internal or external jugular veins, femoral vein, and subclavian vein. Central catheters have considerable risks, and at least one of the routes carries a significant morbidity rate comparable to that of an appendectomy. The internal jugular, subclavian, and femoral vein approaches should not be used in patients with trauma in the region, local infection, previous thrombosis, or any anatomic anomaly. Although systemic infection is a relative contraindication to central venous catheterization (in general, any invasive instrumentation should be avoided in a septic patient), the patient with septic shock requires rapid fluid and medication administration, and peripheral cannulation may prove impossible thereby necessitating central venous catheterization. Shoemaker, supra; Rippe, supra.
To obtain a valid central venous pressure, the catheter tip should be placed within the right atrium or one of the great veins of the thorax (e.g., the superior vena cava, the innominate vein, or subclavian vein). The most desirable site for the catheter tip is the distal innominate or proximal superior vena cava. The right atrium itself should be avoided since many complications can occur at this site including: (1) perforation of the atrial wall with resultant potentially lethal cardiac tamponade (hemorrhage into the pericardium), (2) irritation of the atrial wall producing arrhythmias, and (3) migration of the catheter tip to the inferior vena cava, hepatic veins, right ventricle, or pulmonary arteries Shoemaker, supra; Rippe, supra.
There are a variety of complications that may result from central venous catheterization performed through the antecubital veins. These include sterile phlebitis, infection, and deep venous thrombosis (of subclavian or internal jugular veins). Other rare complications included tamponade, puncture of central veins, air embolism, catheter embolism and limb edema.
The most common complication with internal jugular venous (IJV) cannulation is carotid artery puncture, and unrecognized arterial puncture is potentially life threatening. Accordingly, if there is any doubt whether venipuncture or arterial puncture has been performed, catheters should not be threaded over the guide wire. Nonvascular complications also occur from internal jugular venous catheterization. Pneumothorax and hydro- and hemothorax may rarely occur, as may chylothorax when the left IJV approach is used. Other rare complications include catheter tip migration out of the vasculature, air embolism, erosion of a venous wall and hydrothorax, thrombophlebitis, and pulmonary embolism. Shoemaker, supra; Rippe, supra. While, external jugular venous (EJV) cannulation avoids some of these problems, the success rate of this route is substantially lower than the IJV route.
Three main complications are associated with femoral venous (FV) cannulation: arterial puncture, infection, and thromboembolic events. Inadvertent femoral arterial puncture has been reported in up to six percent of cannulations with most of these occurring in patients with pulseless femoral arteries. Shoemaker, supra; Rippe, supra.
The subclavian venous approach to central venous catheterization has received the greatest attention of all routes used for central venous access However, the rare occurrence of serious or potentially life-threatening complications renders the technique dangerous in clinical situations where complications are more likely to occur (e.g., prior surgery in the subclavian area), or in patients who cannot tolerate the complication of pneumothorax (e.g., patients with chronic obstructive pulmonary disease or acute respiratory distress syndrome).
The complications reported from subclavian venous catheterization and indwelling catheter placement include malposition of catheter tip, local cellulitis, pneumothorax, pleural effusion, hemo- or hydrothorax, subcutaneous emphysema, hydromediastinum, mediastinal hematoma, suppurative mediastinitis, clavicular osteomyelitis, subclavian artery puncture, brachial plexus injury, myocardial perforation with or without tamponade, phrenic nerve injury with diaphragmatic paralysis, arteriovenous malformations, endotracheal tube cuff rupture, catheter embolism, air embolism, infectious complications, venous thrombosis, innominate or SVC perforation, thoracic duct injury with or without chylothorax, venobronchial fistula, and internal mammary artery injury. Shoemaker, supra; Rippe, supra.
Most reports indicate 0 to 5 episodes of infection per 100 catheters, regardless of the route of insertion or length of time the catheter is in place. This assumes that a standardized protocol is used to insert all central lines, and that maintenance of the catheter (bandage changes) is carried out by an "intravenous team," an important factor in reducing the number of such episodes The incidence of catheter-associated infections generally increases proportionally with (1) the duration of the catheter use, (2) number of catheters inserted into the central circulation versus peripheral location, (3) number of catheters placed by surgical cutdown, (4) number of breaks in technique, and (5) number of "unauthorized" manipulations of the catheter. Shoemaker, supra; Rippe supra.
As the foregoing demonstrates, invasive access to monitoring of CVP carries a high risk of both immediate and late complications. Where such catheters are required solely for monitoring trends in CVP rather than for administration of fluids and drugs, non-invasive monitoring is highly preferred, since complications are nil.
A non-invasive visual method for estimating venous pressure is based upon the clinical observation that in normal erect subjects, the veins of the neck are collapsed and cannot be seen, while in patients with congestive heart failure who have elevated venous pressures, the jugular veins located in the neck are distended by the increased pressure within these veins. It has been stated that the vertical distance from the suprasternal notch to the top of the column of blood visible in the external jugular vein reflects the degree of venous pressure elevated above the normal. Burch, supra.
It should be noted, however, that distension of the internal jugular veins, though less obvious than distension of the external jugular veins, more accurately reflects the central venous pressure, as the internal jugular vein is contiguous with the superior vena cava (the site for central venous pressure), and there are no intervening valves to interrupt blood flow. While visualization of the top of the column of blood in the internal jugular vein would therefore provide a more accurate measure of CVP, the internal jugular is typically visualized only with the aid of specialized lighting, and its visualization is difficult even for highly skilled physicians. In many cases, the internal jugular vein is so deep that its blood column cannot be visualized by even the most skilled physicians. This is particularly the case in obese patients and patients with highly developed neck muscles.
It is therefore an object of the invention to provide a non-invasive method and apparatus for measurement of central venous pressure which is not dependent on visual inspection of the jugular veins.
Intracranial pressure, like central venous pressure, has major diagnostic and managerial significance, particularly in neonatal neurological disorders. For example, intracranial pressure may prove useful in evaluating infants with intraventricular hemorrhage, posthemorrhagic hydrocephalus, hypoxic-ischemic encephalopathy, bacterial meningitis, and a variety of other pathological states, and also for evaluating the effects of certain interventions such as external ventricular drainage or lumbar puncture for treatment of posthemorrhagic hydrocephalus. Intracranial pressure alterations per se may lead to adverse consequences by disturbing cerebral blood flow and, less frequently, by shifting neural structures in the brain. J. Volpe, Neurology of the Newborn, 2d Ed., W.B. Saunders Company, 1987.
Since central venous pressure approximates intracranial pressure, monitoring of intracranial pressure also provides information about central venous pressure and, as described above, such information is valuable in the diagnosis and management of a variety of disorders. Salmon et al., The Fontagram: A Noninvasive Intracranial Pressure Monitor, Pediatrics, Vol. 60, No. 5, November, 1977. Ordinarily, intracranial venous pressure is coupled to intracranial pressure since, in order for the veins to remain patent, their pressure must be slightly higher than the surrounding pressure. This coupling has been shown to occur over a wide range of pressures when intracranial pressure is altered. Since the cerebral venous pressure cannot be less than the systemic venous pressure, the cerebral venous pressure and the intracranial pressure only become uncoupled when the latter is lowered below the systemic venous pressure. Welch, The Intracranial Pressure in Infants, Journal of Neurosurgery, Vol. 52, May, 1980.
Though invasive techniques are available, they are not suited to continuous monitoring or even frequent intermittent determinations. Accordingly, non-invasive techniques are preferred. Known non-invasive monitors rely on the anterior fontanel as an indicator of intracranial pressure. More particularly, these devices rely on the applanation principle, which recognizes that when the anterior fontanel is flat, the pressure on both sides of the fontanel is equal, and therefore that the pressure required to maintain the fontanel flat should equal intracranial pressure. Several devices exploiting the applanation principle have been developed, all of which incorporate means for applying a positive pressure to the anterior fontanel sufficient to flatten it and means for measuring that pressure. Volpe, supra, Salamon et al., supra. While some studies have reported good correlation between actual intracranial pressure and intracranial pressure as measured with these devices, it is possible that the pressure required to flatten the fontanel does not match intracranial pressure, as depressing the fontanel may simply bulge other intracranial sites, such as the sutures and the occipital fontanel.
A non-invasive visual approach to measuring intracranial pressure, which also relies on the applanation principle, is disclosed in Welch, supra. With the infant in the dorsal recumbent position, the fontanel is observed and palpated. If it is flat, the vertical distance between the level of the fontanel and the level at which venous pressure is thought to be atmospheric (e.g. the midpoint of the clavicle) is measured, and this height, in centimeters, is said to approximate intracranial pressure in cmH.sub.2 O. If the fontanel is not flat, the head is raised or lowered until the fontanel becomes flat, whereupon the vertical distance measurement is made. The potential inaccuracies of this visual approach are obvious.
A further difficulty of measuring intracranial pressure in infants is that any transducer applied to the infant, particularly at the head, must be both comfortable and secure, otherwise the infant will continuously attempt to remove the transducer, with obvious adverse consequences to long term monitoring.
It is accordingly another object of the invention to provide a method and apparatus for monitoring intracranial pressure, and also to provide an improved transducer therefor which is specifically intended for use with infants.